Surgical tourniquet systems are commonly used to stop the penetration of arterial blood into a portion of a patient's limb, thus creating a clear, dry surgical field that facilitates the performance of a surgical procedure and improves outcomes. A typical surgical tourniquet system of the prior art includes a tourniquet cuff for encircling a patient's limb at a desired location, a tourniquet instrument, and flexible tubing connecting the cuff to the instrument. In some surgical tourniquet systems of the prior art, the tourniquet cuff includes an inflatable bladder that is connected pneumatically to a tourniquet instrument via flexible tubing attached to one or two cuff ports. The tourniquet instrument includes a pressure regulator to maintain the pressure in the inflatable bladder of the cuff near a reference pressure that is above a minimum pressure required to stop arterial blood penetration past the cuff, when applied to a patient's limb at a desired location during a time period suitably long for the performance of a surgical procedure. Many types of such pneumatic surgical tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. Nos. 4,469,099, 4,479,494, 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415 and No. 5,855,589.
Tourniquet cuffs of the prior art are designed to serve as effectors which apply high pressures that stop the penetration of arterial blood past the applied cuff for surgical time periods which can extend from a few minutes to several hours. Tourniquet cuffs of the prior art differ substantially from pneumatic cuffs designed and used for other purposes. For example, pneumatic cuffs employed in the intermittent measurement of blood pressure are typically designed to apply much lower pressures for much shorter periods of time to selected arteries beneath an inflatable bladder portion of the cuff that does not surround the limb; such cuffs must meet standards of design that are fundamentally different from key design parameters of the safest and most effective tourniquet cuffs. Tourniquet cuffs of the prior art are not designed to serve a sensing purpose, and blood-pressure cuffs of the prior art are not designed to serve an effector purpose.
The inward compressive force applied to a limb by a pressurized tourniquet cuff to close underlying arteries is not equal across the width of the cuff, from proximal to distal edges. Consequently when inflated to a minimum pressure required to stop arterial blood flow past the distal edge of the tourniquet cuff, arterial blood within the limb still penetrates beneath the proximal edge of the cuff for some distance to a location where the arteries become closed. In addition to the pneumatic pressure to which a selected tourniquet cuff is inflated, several variables affect the distance to which arterial blood penetrates beneath the cuff. These variables include: the patient's limb characteristics (for example, limb shape, circumference and soft tissue characteristics at the cuff location); characteristics of the selected tourniquet cuff (for example, cuff design, cuff shape and cuff width); the technique of application of the cuff to the limb (for example, the degree of snugness or looseness of application and the absence, presence and type of underlying limb protection sleeve); physiologic characteristics of the patient including blood pressure and limb temperature; the anesthetic technique employed during surgery (for example, whether a general or regional anesthetic is given, the types and dosages of anesthetic agents employed and the degree of attention paid to anesthetic management); the length of time the tourniquet remains inflated on the limb; changes in limb position during surgery; and any shift in the location of the cuff relative to the limb during surgery.
Many studies published in the medical literature have shown that the safest tourniquet pressure is the lowest pressure that will stop the penetration of arterial blood past a specific cuff applied to a specific patient for the duration of that patient's surgery. Such studies have shown that higher tourniquet pressures are associated with higher risks of tourniquet-related injuries to the patient. Therefore, when a tourniquet is used in surgery, surgical staff generally try to use the lowest tourniquet pressure that in their judgment is safely possible.
It is well established in the medical literature that the optimal guideline for setting the pressure of a constant-pressure tourniquet is based on “Limb Occlusion Pressure” (LOP). LOP can be defined as the minimum pressure required, at a specific time in a specific tourniquet cuff applied to a specific patient's limb at a specific location, to stop the flow of arterial blood into the limb distal to the cuff. LOP is affected by variables including the patient's limb characteristics, characteristics of the selected tourniquet cuff, the technique of application of the cuff to the limb, physiologic characteristics of the patient including blood pressure and limb temperature, and other clinical factors (for example, the extent of any elevation of the limb during LOP measurement and the extent of any limb movement during measurement). The currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to the measured LOP, in an effort to account for variations in physiologic characteristics and other changes that may be anticipated to occur normally over the duration of a surgical procedure.
Some surgical tourniquet systems of the prior art include means to measure LOP automatically. Prior-art tourniquet apparatus having automatic LOP measurement means are described by McEwen in U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415. Such prior-art systems have included blood flow transducers that employ a photoplethysmographic principle to sense blood flow in the distal limb, although other transducers have been suggested in the prior art to measure blood flow based on other principles. A blood flow transducer employing the photoplethysmographic principle uses light to indicate the volume of blood present in a transduced region, consisting of a combination of a residual blood volume and a changing blood volume resulting from arterial pulsations. An additional predetermined pressure margin based on recommendations in published surgical literature is added to the automatically measured LOP to provide a “Recommended Tourniquet Pressure” (RTP), as a guideline to help the surgical staff select the lowest tourniquet pressure that will safely stop arterial blood flow for the duration of a surgical procedure. Such prior-art systems allow the surgical staff to select the RTP based on LOP as the tourniquet pressure for that patient, or to select another pressure based on the physician's discretion or the protocol at the institution where the surgery is being performed.
Despite the improved performance of prior-art apparatus that automatically measures LOP, there are three significant limitations. The first limitation is that a separate, complex and costly distal flow sensor is required: the correct application and use of the required distal sensor for automatic LOP measurement is dependent on the skill, training and experience of the surgical staff; the sensor must be located distally on the limb undergoing surgery and this may not be possible in some instances; in other instances the distal location of the sensor requires placement of a non-sterile sensor in or near a sterile surgical field and interferes with the pre-surgical preparation of the limb, thus disrupting the pre-surgical workflow and undesirably increasing the overall perioperative time and costs. A second limitation is that the apparatus of the prior art does not measure or estimate any changes to LOP that may occur during surgery. The third limitation is that the Recommended Tourniquet Pressure (RTP) is not a personalized tourniquet pressure (PTP) for that individual patient, and instead is a population estimate equaling the sum of the LOP measured at some time pre-surgically plus a population-based and predetermined increment of pressure. This increment is set to be an increment greater than the magnitude of an increase in LOP normally expected during surgery, but the amount of increment is based on aggregated data from a population of surgical patients during a wide variety of surgical procedures and is not personalized to an individual patient undergoing a specific surgical procedure under a specific anesthetic protocol. Accordingly, an RTP of the prior art is not a PTP, and may be higher or lower than optimal.
In U.S. Pat. No. 6,605,103 Hovanes et al. describe apparatus for detecting the flow of blood past a tourniquet cuff and into a surgical field. Such prior-art apparatus is impractical because blood must flow past the tourniquet cuff before it can be detected, requiring surgical staff to do one of two things if blood enters the surgical field: interrupt the surgical procedure and take action to remove the blood; or proceed with blood in the field which might affect visualization and the quality of surgery. Further, Hovanes et al. relies on the accurate sensing of the onset of blood flow past a tourniquet cuff by the measurement of blood flow-related signals. Such apparatus can only be used when arterial blood is actually flowing past the tourniquet cuff toward the surgical field.
Certain prior-art systems adapt ultrasonic Doppler techniques to sense the penetration of arterial blood within a portion of a limb beneath an encircling tourniquet cuff. Examples of such systems are described by McEwen and Jameson in U.S. Pat. No. 8,366,740 and US Patent Publication No. 2013/0144330, and by McEwen et al in US Patent Publication No. 2013/0190806. Detection of arterial blood penetration within a limb beneath a tourniquet cuff by adapting ultrasonic Doppler apparatus and methods requires the accurate measurement of small pulsatile signals in the presence of relatively large levels of noise, especially as the amount of arterial blood beneath the cuff decreases. Further, detection of blood penetration by such methods must be rapid as well as accurate, to facilitate dynamic and accurate control of tourniquet pressure during surgery. Ultrasonic tourniquet systems of the prior art have other significant limitations: the additional ultrasonic sensing arrays required, together with the associated ultrasonic signal processing circuitry and software, are costly; also, adapting and incorporating ultrasonic sensing arrays into tourniquet cuffs is complex and costly, and may be prohibitive in view of the fact that competing tourniquet systems employ cuffs that are sterile, low-cost, disposable products; further, the safe operation of ultrasonic tourniquet systems is at present complex and user-dependent, requiring additional user training and skill.
There is a need for a tourniquet system that can establish and maintain a tourniquet pressure that is personalized for each surgical patient, and optimized for each surgical procedure, and for each applied tourniquet cuff. Preferably, such a system would be implemented without the need for substantially increased training, knowledge or skill on the part of the surgical staff. There is also a need for a personalized tourniquet system that overcomes the requirement of the prior art for a separate, complex and costly distal blood flow sensor or other apparatus for estimating the patient's limb occlusion pressure before surgery. Such a system would also overcome the requirement of the prior art for separate, costly and complex apparatus to sense, display, monitor and control the distance of penetration of arterial blood within the limb beneath an applied tourniquet cuff during surgery. There is a related need for a personalized tourniquet system having a dual-purpose tourniquet cuff wherein the same inflatable bladder of the tourniquet cuff can be separately operated as a patient sensor or as a tourniquet effector, or simultaneously operated as a combined sensor and effector.